1. Field of the Invention
The present invention relates to an air-fuel ratio control system for an internal combustion engine having a plurality of cylinders, and particularly to a control system which can determine an imbalance failure that air-fuel ratios corresponding a plurality of cylinders in the engine differ with each other more greatly than the allowable limit.
2. Description of the Related Art
Japanese Patent Laid-open Publication No. 2011-144754 (JP-'754) discloses an air-fuel ratio control system which can determine the imbalance failure based on the output signal of the air-fuel ratio sensor disposed in the exhaust system of the engine. According to this system, the air-fuel ratio perturbation control is performed during the engine operation for oscillating the air-fuel ratio with a predetermined frequency, and the imbalance failure is determined using a ratio parameter which is obtained during the perturbation control. The ratio parameter is calculated by dividing an intensity of the 0.5th-order frequency component contained in the output signal of the air-fuel ratio sensor by an intensity of the predetermined frequency component contained in the output signal of the air-fuel ratio sensor. The 0.5th-order frequency component is a component of a frequency which is half of the frequency corresponding to the engine rotational speed. When the imbalance failure occurs, the 0.5th-order frequency component intensity increases, and a value of the ratio parameter increases as the degree of the imbalance failure increases. Accordingly, the imbalance failure can be determined by comparing the ratio parameter with a predetermined threshold value.
According to the system shown in JP-'754, the amplitude of the air-fuel oscillation in the air-fuel ratio perturbation control is fixed to a constant value. Accordingly, the determination accuracy may deteriorate depending on the imbalance degree of the actual air-fuel ratio. The imbalance in the air-fuel ratios occurs, for example, when any one of the fuel injection valves disposed in the plurality of cylinders fails. The imbalance degree therefore can be expressed, for example, with a deviation ratio from the normal value of the fuel injection amount of one fuel injection valve.
FIG. 13A is a graph for explaining this problem. The horizontal axis of FIG. 13A corresponds to an execution number NDET of the determination, and the vertical axis corresponds to the ratio parameter RT. The data group DG1 corresponds to a state where there is no imbalance of the air-fuel ratios, the data group DG2 corresponds to a low-degree imbalance state where the fuel injection amount of one cylinder deviates from the normal value by 10%, and the data group DG3 corresponds to a high-degree imbalance state where the fuel injection amount of one cylinder deviates from the normal value by 40%.
Regarding the data groups DG1 and DG2, the distribution width of values of the ratio parameter RT is comparatively narrow, whereas the distribution width corresponding to the data group DG3 is very wide. Accordingly, in the example shown in FIG. 13A, accuracy of the ratio parameter RT in the high-degree imbalance state decreases, so that a possibility of incorrect determination becomes higher. In FIG. 13A, the data groups DG1 and DG2 are shown as black areas, since the obtained data points exist in a narrow area.